1. Field of the Invention
The present invention relates to a motor controller that performs tandem control in which one driven body is driven by two motors.
2. Description of the Related Art
In a driving mechanism such as a robot or machine tool, when the movement axis cannot be accelerated or decelerated by one motor because a driven body to be moved is too big or when a driven body cannot be moved stably because backlash between the motor and the driven body is large, tandem control in which two motors receive commands and drive one driven body is used. There are two types of tandem control: torque tandem control and position tandem control.
U.S. Pat. No. 5,646,495 discloses the stabilization (dumping correction) of torque tandem control in which one axis is driven by two motors and the addition of a preload for suppression of backlash. FIG. 14 is a block diagram illustrating principal parts of a motor controller for the torque tandem control. The main motor (not shown) and sub motor (not shown) are driven by voltage commands from corresponding current control units.
The main motor is controlled by inputting a torque command Tc1′, which is obtained by adding a preload torque Tp1′ to a torque command sent from a velocity control unit, to a current control unit 13 of the main motor. The sub motor is controlled by adding a preload torque Tp2′ to a torque command sent from the velocity control unit and inputting a torque command Tc2′ obtained through a reverse unit to a current control unit 23 of the sub motor.
The preload torque Tp1′ and the preload torque Tp2′ are torque offsets to be added to torque commands computed and output by the velocity control units so as to maintain tension between one gear driven by the main motor and the other gear driven by the sub motor.
Japanese Patent Application Laid-Open No. 7-110714 discloses a method of controlling a position, velocity, and torque when one movable member is driven by a plurality of motors. Of the plurality of motors, one is a servo motor, which controls the position and velocity of the movable member, and the others are motors that can control torque, which are driven by a set torque command given.
Japanese Patent Application Laid-Open No. 2003-79180 discloses a motor controller that performs tandem control in which one movable part is driven by a master axis motor and a slave axis motor, wherein a torque arbitration value is computed by low-pass filtering of a difference between a torque command for the master axis motor and a torque command for the slave axis motor. The torque arbitration value obtained by the computation is added to the torque command for the slave axis motor, so that the torque command for the slave axis motor is gradually corrected to the torque command for the master axis motor.
U.S. Pat. No. 6,809,493 discloses a servo controller that performs tandem control in which one driven body is driven by a plurality of motors. As shown in FIG. 13, this servo controller includes position control units 11 and 21, velocity control units 12 and 22, current control units 13 and 23, current amplifiers 14 and 24, and velocity detector 17 and 27, for each motor. The servo controller further includes a velocity integrator sharing means 5 that makes the output value of an integral element of the velocity control unit 12 identical to the output value of an integral element of the velocity control unit 22. The same command is input to the control systems for two motors 15 and 25 used for tandem control and the output values of the integral elements of the velocity control units 12 and 22 are kept substantially identical to each other by the velocity integrator sharing means 5.
In the technique for adding a preload torque disclosed in U.S. Pat. No. 5,646,495, when backlash between gears of a tool machine or the like is large, a preload torque is added to the master motor and slave motor in torque tandem control to suppress the backlash. However, the preload torque is added in a step-like manner in this patent document, so a mechanical shock may occur during addition or removal of a preload torque.
As shown in FIG. 15, even if a preload torque is added in position tandem control, the reciprocal of the preload torque is accumulated in the velocity integrator and the output torque becomes 0. Therefore, it is impossible to suppress the backlash between gears.